1. Field of the Invention
The present invention relates to a process for preparing a permeable adhesive tape. More specifically, the present invention relates to an industrially advantageous process for preparing a permeable adhesive tape which comprises making holes with the aid of water, or water and absorptive high-molecular weight compound.
2. Description of the Related Art
The present invention is specifically directed to adhesive tapes used for protecting wounds or others. By way of example, the adhesive tapes referred to herein are meant to include adhesive bandages, sticking plasters and bandages, surgical cloths. Such adhesive tapes cause a rash when adhered and maintained on skin for a long time. For avoiding this problem, it is desirous that tapes have permeability. In order to solve this problem, it is known to form holes in tapes by using the following hole forming methods:
(i) mechanically, for instance, by punching the product;
(ii) chemically, for instance, by incorporating a substance which can be leached out by a solvent from the product;
(iii) physically, for instance, by causing the product to foam by expelling a solvent during drying, thus making the material microporous; and
(iv) by physical interaction between the backing material and the adhesive, for example, by allowing the adhesive to dry in contact with a fabric backing material which causes the adhesive to become porous.
Examples of known products classified in (iii) are "Micropore" (trademark of 3M in USA), and tapes disclosed in Japanese Unexamined Patent Publication No. 57(1982)-182375 and the like. For instance, the above No. 57(1982)-182375 discloses that an adhesive layer is formed by using a dispersion of an adhesive agent in an organic solvent, and is given for permeability therein.
In this regard, a process for conveniently and efficiently preparing an adhesive tape having high permeability is disclosed in U.S. Pat. No. 4,789,413 which comprises dispersing water, or water and a water absorptive high-molecular weight compound homogeneously in a solution of a natural or synthetic rubber or acrylic pressure sensitive adhesive agent in an organic solvent, applying the dispersion onto a releasing agent applied sheet, drying the adhesive agent applied sheet, and laminating a porous material on the adhesive agent applied surface to obtain a permeable adhesive tape. This process will hereinafter be termed as the "water dispersion method".
The present invention is to improve the above water dispersion method. Particularly, this invention is to provide a permeable adhesive tape incorporating an adhesive layer being improved in both its tenacity after adhering to an object (breaking strength) and its adhesion to a porous material (peel strength).
A generally known method to improve a pressure sensitive adhesive agent in tenacity is adding an isocyanate crosslinker to the pressure sensitive agent in such an amount as not to hinder the adhesion thereof. Therefore, with the above water dispersion method, an adhesive layer, is formed by using a modified dispersion containing an isocyanate crosslinker which is specifically prepared by adding the crosslinker to the organic solution of a pressure sensitive adhesive agent used as a dispersion medium of the above dispersion.
In this case, however, the added isocyanate crosslinker reacts with water coexisting therewith to gelate the dispersion, so that homogeneous application, or application itself of the dispersion, becomes impossible. That results in difficulty in forming the adhesive layer.
To avoid this gelation, adding the isocyanate crosslinker to the dispersion just before the application can be theoretically inferred. Even in this situation, however, the added isocyanate crosslinker is consumed by reacting with water, with the result that it is still difficult to sufficiently improve tenacity and adhesion to the porous material.